The Group of Seven most developed economies is calling on Russia to stop all efforts to "annex" Ukraine's Crimea region, European Commission President Jose Manuel Barroso said Wednesday.
G7 leaders, European Council head Hermann Van Rompuy "and I will in a new declaration call on Russia to cease all efforts to annex Ukraine's autonomous republic of Crimea," Barroso said in a tweeted message.
"Together with other G7 leaders, Van Rompuy and myself have strongly and unequivocally condemned this action on behalf of the EU," Barroso said.
The announcement will be made at 1300 GMT, he added.
Earlier this month, the G7 countries condemned Russia, their partner in the G8, for its "clear" violation of Ukraine's sovereignty and suspended preparations for a June G8 summit in Sochi to be hosted by President Vladimir Putin.
They said Russia's actions in Ukraine were incompatible with its role in the G8, which it joined in 1997 as Moscow returned to the world stage with great fanfare after the fall of Communism.
The G7 -- Britain, Canada, France, Germany, Italy, Japan, the United States -- were joined by Barroso and Van Rompuy in that statement last week.
Since then, Russia has tightened its hold on the Crimea, home to a large Russian-speaking population and the base for its Black Sea fleet.
Pro-Russian leaders there have organised a referendum for Sunday on whether Crimea should join with Russia, with most expecting the vote, if it goes ahead, to produce a large majority in favour.
Barroso said the referendum was illegal and called for immediate steps to de-escalate the situation.
"Any attempt to legitimise a referendum in Crimea is contrary to the Ukrainian constitution and international law and quite clearly illegal," he said
"If meaningful negotiations do not begin within the next few days and produce results within a limited timeframe, this will trigger additional measures," he added.
The G7 announcement will be made as Ukraine interim prime minister Arseniy Yatsenyuk heads for talks with US President Barack Obama, with the pressure for more sanctions against Moscow mounting.